The use of bone grafts and bone substitute materials in orthopedic medicine is known. While bone wounds can regenerate without the formation of scar tissue, fractures and other orthopedic injuries take a long time to heal, during which time the bone is unable to support physiologic loading unaided. Metal pins, screws, rods, plates and meshes are frequently required to replace the mechanical functions of injured bone. However, metal is significantly more stiff than bone. Use of metal implants may result in decreased bone density around the implant site due to stress shielding. Physiologic stresses and corrosion may cause metal implants to fracture. Unlike bone, which can heal small damage cracks through remodeling to prevent more extensive damage and failure, damaged metal implants can only be replaced or removed. The natural cellular healing and remodeling mechanisms of the body coordinate removal of bone and bone grafts by osteoclast cells and formation of bone by osteoblast cells.
Conventionally, bone tissue regeneration is achieved by filling a bone repair site with a bone graft. Over time, the bone graft is incorporated by the host and new bone remodels the bone graft. In order to place the bone graft, it is common to use a monolithic bone graft or to form an osteoimplant comprising particulated bone in a carrier. The carrier is thus chosen to be biocompatible, to be resorbable, and to have release characteristics such that the bone graft is accessible. Generally, the formed implant, whether monolithic or particulated and in a carrier, is substantially solid at the time of implantation and thus does not conform to the implant site. Further, the implant is substantially complete at the time of implantation and thus provides little ability for customization, for example by the addition of autograft.
The use of bone grafts is generally limited by the available shape and size of grafts. Bone grafts using cortical bone remodel slowly because of their limited porosity. Traditional bone substitute materials and bone chips are more quickly remodeled but cannot immediately provide mechanical support. In addition, while bone substitute materials and bone chips can be used to fill oddly shaped bone defects, such materials are not as well suited for wrapping or resurfacing bone.
Bone graft materials are often used in spine fusion surgery. Current spinal fusion implants utilize grafts of either bone or artificial implants to fill the intervertebral disc space.
Interspinous process devices have been developed as a minimally invasive surgical option for treating conditions such as lumbar spinal stenosis. A spacer fits between the spinous processes in the back of the spine. Its role is to keep the space for the nerves open by spreading the vertebrae apart.
As to bone grafts, allograft bone is a reasonable bone graft substitute for autologous bone. It is readily available from cadavers and avoids the surgical complications and patient morbidity associated with harvesting autologous bone. Typically, allograft bone is a load-bearing matrix comprising cross-linked collagen, hydroxyapatite, and osteoinductive bone morphogenetic proteins. Human allograft tissue is widely used in orthopedic surgery.
Indeed, allograft is a preferred material by surgeons for conducting interbody fusions because it will remodel over time into host bone within the fusion mass. Current concepts of using allograft implants in an interspinous process fusion involve fully mineralized pieces of solid cortical allograft. However, though allograft tissue has certain advantages over other treatments, allograft implants have some limitations, e.g., it is typically available in only limited size ranges, thus making it difficult to provide a perfect fit to the geometry of the spine in particular, to the geometry of the spinous processes. This could significantly increase the amount of time necessary for bone fusion to occur. Indeed, a conventional cortical allograft implant may have an improper fit between the cortical allograft implant and the spinous processes resulting in several gaps that prevent successful bone fusion from occurring.
Therefore, it would be desirable to construct an implant, particularly an interspinous implant device, which enhances allograft treatment.